When radiation-sensitive silver halide emulsions differing in mean grain size are optimally sensitized, there is a predictable relationship between photographic speed and granularity. It is generally recognized that each doubling of photographic speed results in an increase of 5-7 granularity units. When emulsions of differing speed also differ in granularity by a predicted number of granularity units, the emulsions are said to exhibit the same speed-granularity relationship. An emulsion which shows increased speed without a proportional increase in granularity units is not only a faster emulsion, but an emulsion exhibiting a superior speed-granularity relationship. An emulsion which exhibits reduced granularity without a proportionate loss of speed also exhibits an improved speed-granularity relationship.
Kofron et al U.S. Pat. No. 4,439,520 teaches that substantially optimally sensitized high aspect ratio tabular grain emulsions are capable of exhibiting improved speed-granularity relationships and other significant photographic advantages. Kofron et al in column 41, line 42, through column 42, line 21, discloses employing high aspect ratio tabular grain emulsions in combination with conventional emulsions, either by blending the emulsions or by coating the emulsions in separate layers. Kofron et al recognizes that when a fine grain silver chloride emulsion is blended with a high aspect ratio tabular grain emulsion an improvement in the speed-granularity relationship of the blended emulsion can be realized. In general, however, Kofron et al teaches that increased photographic speed occurs when faster and slower silver halide emulsions are coated in separate layers as opposed to blending.
Silver bromide and silver bromoiodide emulsions are almost invariably selected for photographic and radiographic applications requiring the highest levels of photographic speed. Notwithstanding the advances in the art which Kofron et al represents, there has remained a need for silver bromide and bromoiodide emulsions with still better speed-granularity relationships.